Rapid adaptation of primate LGN neurons to drifting grating stimulation.

The visual system needs to dynamically adapt to changing environments. Much is known about the adaptive effects of constant stimulation over prolonged periods. However, there are open questions regarding adaptation to stimuli that are changing over time, interrupted, or repeated. Feature-specific adaptation to repeating stimuli has been shown to occur as early as primary visual cortex (V1), but there is also evidence for more generalized, fatigue-like adaptation that might occur at an earlier stage of processing. Here, we show adaptation in the lateral geniculate nucleus (LGN) of awake, fixating monkeys following brief (1 s) exposure to repeated cycles of a 4-Hz drifting grating. We examined the relative change of each neuron's response across successive (repeated) grating cycles. We found that neurons from all cell classes (parvocellular, magnocellular, and koniocellular) showed significant adaptation. However, only magnocellular neurons showed adaptation when responses were averaged to a population response. In contrast to firing rates, response variability was largely unaffected. Finally, adaptation was comparable between monocular and binocular stimulation, suggesting that rapid LGN adaptation is monocular in nature.NEW & NOTEWORTHY Neural adaptation can be defined as reduction of spiking responses following repeated or prolonged stimulation. Adaptation helps adjust neural responsiveness to avoid saturation and has been suggested to improve perceptual selectivity, information transmission, and predictive coding. Here, we report rapid adaptation to repeated cycles of gratings drifting over the receptive field of neurons at the earliest site of postretinal processing, the lateral geniculate nucleus of the thalamus.

Stimulating both eyes with matching stimuli enhances V1 responses.

Neurons in the primary visual cortex (V1) of primates play a key role in combining monocular inputs to form a binocular response. Although much has been gleaned from studying how V1 responds to discrepant (dichoptic) images, equally important is to understand how V1 responds to concordant (dioptic) images in the two eyes. Here, we investigated the extent to which concordant, balanced, zero-disparity binocular stimulation modifies V1 responses to varying stimulus contrast using intracranial multielectrode arrays. On average, binocular stimuli evoked stronger V1 activity than their monocular counterparts. This binocular facilitation scaled most proportionately with contrast during the initial transient. As V1 responses evolved, additional contrast-mediated dynamics emerged. Specifically, responses exhibited longer maintenance of facilitation for lower contrast and binocular suppression at high contrast. These results suggest that V1 processes concordant stimulation of both eyes in at least two sequential steps: initial response enhancement followed by contrast-dependent control of excitation.